1. Field of the Invention
This invention relates generally to the field of geophysical prospecting. More particularly, the invention relates to the field of marine seismic surveying with towed streamers.
2. Description of the Related Art
In the oil and gas industry, geophysical prospecting is commonly used to aid in the search for and evaluation of subterranean formations. Geophysical prospecting techniques yield knowledge of the subsurface structure of the earth, which is useful for finding and extracting valuable mineral resources, particularly hydrocarbon deposits such as oil and natural gas. A well-known technique of geophysical prospecting is a seismic survey. In a land-based seismic survey, a seismic signal is generated on or near the earth's surface and then travels downward into the subsurface of the earth. In a marine seismic survey, the seismic signal may also travel downward through a body of water overlying the subsurface of the earth. Seismic energy sources are used to generate the seismic signal which, after propagating into the earth, is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, specifically sound wave velocity and rock density, which lead to differences in acoustic impedance at the interfaces. The reflected seismic energy is detected by seismic sensors (also called seismic receivers) at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes and recorded.
The appropriate seismic sources for generating the seismic signal in land seismic surveys may include explosives or vibrators. Marine seismic surveys typically employ a submerged seismic source towed by a ship and periodically activated to generate an acoustic wavefield. The seismic source generating the wavefield may be of several types, including a small explosive charge, an electric spark or arc, a marine vibrator, and, typically, a gun. The seismic source gun may be a water gun, a vapor gun, and, most typically, an air gun. Typically, a marine seismic source consists not of a single source element, but of a spatially-distributed array of source elements. This arrangement is particularly true for air guns, currently the most common form of marine seismic source.
The appropriate types of seismic sensors typically include particle velocity sensors, particularly in land surveys, and water pressure sensors (typically pressure gradient sensors), particularly in marine surveys. Sometimes particle acceleration sensors are used in place of or in addition to particle velocity sensors. Particle velocity sensors and water pressure sensors are commonly known in the art as geophones and hydrophones, respectively. Seismic sensors may be deployed by themselves, but are more commonly deployed in sensor arrays. Additionally, pressure sensors and particle velocity sensors may be deployed together in a marine survey, collocated in pairs or pairs of arrays.
The resulting seismic data obtained in performing the survey is processed to yield information relating to the geologic structure and properties of the subterranean formations in the area being surveyed. The seismic data is processed for display and analysis of potential hydrocarbon content of these subterranean formations. The goal of seismic data processing is to extract from the seismic data as much information as possible regarding the subterranean formations in order to adequately image the geologic subsurface. In order to identify locations in the Earth's subsurface where there is a probability for finding petroleum accumulations, large sums of money are expended in gathering, processing, and interpreting seismic data. The process of constructing the reflector surfaces defining the subterranean earth layers of interest from the recorded seismic data provides an image of the earth in depth or time.
The image of the structure of the Earth's subsurface is produced in order to enable an interpreter to select locations with the greatest probability of having petroleum accumulations. To verify the presence of petroleum, a well must be drilled. Drilling wells to determine whether petroleum deposits are present or not, is an extremely expensive and time-consuming undertaking. For that reason, there is a continuing need to improve the processing and display of the seismic data, so as to produce an image of the structure of the Earth's subsurface that will improve the ability of an interpreter, whether the interpretation is made by a computer or a human, to assess the probability that an accumulation of petroleum exists at a particular location in the Earth's subsurface.
In a typical marine seismic survey, a seismic survey vessel travels on the water surface, typically at about 5 knots, and contains seismic acquisition equipment, such as navigation control, seismic source control, seismic sensor control, and recording equipment. The seismic source control equipment causes a seismic source towed in the body of water by the seismic vessel to actuate at selected times. Seismic streamers, also called seismic cables, are elongate cable-like structures towed in the body of water by the seismic survey vessel that tows the seismic source or by another seismic survey ship. Typically, a plurality of seismic streamers are towed behind a seismic vessel. The seismic streamers contain sensors to detect the reflected wavefields initiated by the seismic source and reflected from reflecting interfaces. Conventionally, the seismic streamers contain pressure sensors such as hydrophones, but seismic streamers have been proposed that contain water particle velocity sensors such as geophones or particle acceleration sensors such as accelerometers, in addition to hydrophones. The pressure sensors and particle motion sensors may be deployed in close proximity, collocated in pairs or pairs of arrays along a seismic cable.
Unfortunately, several asymmetrical features are involved in conventional 3D marine seismic acquisition by towed streamer. These features include asymmetrical spatial sampling and fold between inline and cross-line directions, asymmetrical illumination due to single direction shooting, and asymmetrical offsets between streamer length and streamer span width. Thus, the single-direction shooting in towed streamer acquisition typically results in a regular but asymmetrical array of seismic data sampling points, such as Common Mid Points (CMP's). The sampling density of the sampling points is denser in the inline direction (parallel to the towed streamers) than in the cross-line direction (perpendicular to the towed streamers). The asymmetry is due to a wider spacing between receivers in separate streamers than between receivers in the same streamer.
Some new marine acquisition technologies have been developed recently to address and mitigate the above weakness of the conventional towed streamer seismic systems. These technologies include High Density (HD) surveys, Multi-Azimuth Towed Streamer (MATS) surveys, and Wide-Azimuth Towed Streamer (WATS) surveys. The first of these technologies, HD, attempts to solve the asymmetrical sampling issue while the latter two, MATS and WATS, attempt to address the asymmetrical illumination and offsets issues. All these technologies achieve signal-to-noise ratio improvement in the same way by acquiring and stacking more data for the same area.
HD technology is still a single-direction shooting method. HD technology addresses the asymmetrical sampling issue by reducing separation of streamers and increasing the cross-line density of streamer passes. For instance, reducing the separation of streamers to half or doubling the number of streamer passes would double the cross-line density of sampling points. For HD surveys, twice the streamer passes, at roughly twice the cost, gives twice the sampling density as conventional shooting.
Multi-Azimuth Towed Streamer technology is a multi-direction shooting method. MATS addresses the asymmetrical illumination issue by increasing the number of directions (azimuths) that streamer passes are made in. Increasing the azimuths increases the illumination of the seismic targets, but does not, in general, increase the sampling density. For MATS, twice the streamer passes, at roughly twice the cost, gives increased illumination, but the same sampling density as conventional shooting. Thus, for the same cost, HD delivers denser spatial sampling than MATS. Additional cost for more azimuths and sparser spatial sampling than HD are two main constraints preventing multi-azimuth techniques such as MATS from being a more popular exploration tool.
Thus, a need exists for a method for acquiring marine towed streamer seismic data that improves both the spatial sampling density and the illumination over convention acquisition techniques. In particular, a need exists for a method for processing multi-azimuth seismic data that yields greater spatial sampling density than conventional processing does.